Abstract

A short polarization beam splitter (PBS) is presented based on an asymmetrical
evanescent coupling system, which consists of a narrow input waveguide, a narrow output
waveguide, and a wide middle optical waveguide between them. The width of the waveguides
is designed so that the phase-matching condition is satisfied for the TM fundamental
(TM<sub>0</sub>) mode in the narrow input/output waveguide and the first higher order
TM (TM<sub>1</sub>) mode in the wide middle waveguide. Meanwhile, there is a significant
phase mismatch for the case with TE polarization. Therefore, for the launched TE
polarized light, almost no coupling happens when it goes through the coupling region and
finally the TE polarized light is output from the through port. For the launched
TM<sub>0</sub> mode in the narrow input waveguide, it is completely coupled to the
TM<sub>1</sub> mode in the wide middle waveguide by choosing the optimal length of the
coupling region. Furthermore, the TM<sub>1</sub> mode excited in the wide middle
waveguide is then coupled to the TM<sub>0</sub> mode in the narrow output waveguide
through the evanescent coupling between them. A short (~25μm long) PBS is
designed based on silicon-on-insulator nanowires, while the gap width is chosen as large
as 300 nm to make the fabrication easy. Numerical simulations show that the present PBS
has a good fabrication tolerance for the variation of the waveguide width (more than
±20 nm) and a broadband (~50 nm) for an extinction ratio of >15 dB.